Paper and paperboard products have many uses in which it is important that the paper be free from odor and taste. For example, most consumer packaging, and especially food packaging, requires paper that is substantially free from unpleasant odors and tastes.
Unfortunately, conventional paper making processes provide ample opportunity for the paper to acquire odor. For example, odors may derive from the microbiological activity which is pervasive in pulp processing systems. Many of the additives used in paper manufacturing and finishing can also impart unpleasant odors to the paper, as can the degradation products of those additives. Additionally, the oxidation of unsaturated fatty acids (UFAs) is known to lead to the production of volatile organic compounds, which often have an unpleasant smell.
The practice of chlorine pulp bleaching, commonly using 30 to 50 kilograms of chlorine (expressed as Cl.sub.2) per metric ton of paper produced, has long been used in the art to clean the pulp stock, and that bleaching has provided the additional benefit of controlling odors as well. Recently however, many paper and paperboard producers have become reluctant to practice chlorine bleaching--due largely to environmental reasons.
To replace the old bleaching technology, a variety of more environmentally friendly technologies have been developed. For example, microbiological activity can effectively be controlled using low levels of oxidizing biocide (typically 50-100 g/metric ton, expressed as Cl.sub.2) in the thinstock cycles and water recycling systems where the activity is most problematic. Proper additive selection is generally the best way to mitigate odors associated with additives, although low levels of biocides are occasionally used to minimize additive odors as well.
Unfortunately however, the volatile organic compounds produced from the degradation of unsaturated fatty acids cannot be controlled using these techniques. The oxidative degradation of these organic compounds occurs gradually over time, including during storage of finished products, and the addition of low levels of biocide as described above does not prevent that degradation. A need therefore exists for a method of controlling the production of volatile organic compounds, particularly those that arise from the breakdown of unsaturated fatty acids.
As further background to the invention, it is known that resins present in wood are the primary source of UFA in the finished paper or paperboard. Some additives, such as anti-foaming agents, may also contribute UFAs to a lesser extent.
Linoleic acid is the most prevalent UFA in wood resins, followed by oleic acid and linolenic acid. These UFAs oxidize naturally over time, and the autooxidation of these compounds yields volatile compounds with unpleasant odors or tastes. Hexanal is perhaps the most common of these breakdown products.
It is believed that the UFA autooxidation proceeds by a free radical mechanism, possibly catalyzed by metals. For this reason, it has been proposed that chelants may help reduce the rate of UFA autooxidation. Unfortunately, the practical aspects of that approach have not yet been established.
Much of the UFA present in pulp is associated with the fiber, making rigorous pulp washing one method for removing UFAs. Fiber washing requires large quantities of fresh water however, increasing production costs and making this approach impractical for large-scale commercial applications.
Anti-oxidants, such as BHT and BHA, can sometimes be used to reduce the rate of auto-oxidation in foods. Here too though, the practical aspects of this approach in commercial production processes have not yet been determined.
In view of the above, it can be seen that a need currently exists for a method of preventing the breakdown of unsaturated fatty acids in paper and paperboard products, thereby controlling the resulting odors. The present invention addresses that need.